Separation of nickel and cobalt from calcium, magnesium and manganese by solvent extraction with synergistic mixtures of carboxylic acids

The potential use of synergistic mixtures of reagents for the solvent extraction of nickel (and cobalt) from lateritic leach liquors was pointed out in the early 1980s by Grinstead, who used dinonylnaphthalene sulphonic acid in conjunction with some 2-pyridyl compounds for the recovery of these metals from slightly acidic solutions1,2. Shortly thereafter, studies at Mintek revealed marked synergistic effects in the extraction of nickel by organophosphoric acids in the presence of certain nonchelating oximes3,4. Subsequently, we extended our work to include mixtures of carboxylic acids and nonchelating oximes, and it was shown that several potentially useful separations between valuable and gangue metals could be obtained, for example pH50Ca – pH50Ni = 3.5 pH units, and pH50Mn – pH50Co = 1.5 pH units5. This work was patented in several countries during the early 1980s6, but the depressed state of the nickel market at that time discouraged any further development of these extractants. Some time later, however, when renewed interest began to be expressed in the recovery of nickel from laterite sources7, we resumed our studies on mixed extractant systems, and chose to use synergists different from the less acid-resistant oximes used previously8. In continuing studies9–11, particular attention was given to the separation of nickel from calcium since, in commercial operations, the low solubility of the sulphate salt of the latter metal can result in the troublesome precipitation of gypsum (CaSO4.2H2O), not only in the stripping and scrubbing circuits (where high organic-to-aqueous phase ratios are desirable in order to upgrade the concentration of nickel from its value in the leach liquor), but sometimes in the extraction circuit as well12. The separation of cobalt and manganese is also of consideration, as this would allow recovery of nickel and cobalt from sulphate leach liquors, whilst effectively rejecting the manganese, calcium and magnesium. These extraction systems are characterized by substantial synergistic shifts (∆pH50) in the Separation of nickel and cobalt from calcium, magnesium and manganese by solvent extraction with synergistic mixtures of carboxylic acids